Lead is the most well-known metal among those emitted by road traffic; even though over a decade has passed since leaded petrol was banned, the presence of this contaminant remains on the edges of roads. Nevertheless, Carrero has confirmed that despite its great toxicity, there are other metals that can pose even greater risks for the environment, due to the fact that they can filter through deeper soil layers. Apart from lead, this researcher has studied zinc oxide (produced by tyre wear) and barium, copper and antimony (resulting from brake pad wear), for example.

The older they are, the more there is

Carrero has extracted the metals found in the samples taken and has determined the concentration in which most of them occur. It can be seen that there is a greater accumulation of metals linked to traffic in the upper levels of the soil closest to the veteran roads. “It depends on how long the road has been in use. It is in the older roads that the metals accumulate and a gradient of concentration with the depth can be seen, while in the remainder of the spots an accumulation like this has not yet had time to develop," explains the researcher.

Likewise, the presence of zinc is well-known, since the deterioration of the zinc plating covering the crash barriers leads to a high concentration of this contaminant in the soil immediately below.

Toxicity is not the whole story

The preliminary studies were followed up by a Raman spectroscopy analysis, which is used to identify the molecular forms in which the compounds are found. Knowing these forms helps to determine the danger posed by each metal for the environment. And the fact is, as Carrero explains, the actual toxicity of each metal is not the only factor to bear in mind: “It matters whether the metal is in the form of a carbonate, or a nitrate... They are different salts with different solubility. The metals can be retained in the soil, so the toxicity would not be as significant as if they had been in the form of a more soluble substrate, in which case, this substrate could be dissolved by the rainwater and move to other compartments. Then, the toxicity could move to underground water tables or be taken up by plants.”

In connection with this, Carrero has simulated the actual conditions that are produced in the environment to observe how the metals react in their presence and whether they become more soluble and, therefore, more hazardous. It turns out that barium and zinc are the metals that need to be closely monitored: “They are emitted as a result of tyre or brake wear, and they are accumulating in the form of oxide in the upper layers, and this has less mobility. However, we have seen that they react with atmospheric CO2 and form carbonates, which are more soluble and permeate. In fact, we have come across a great concentration of carbonate in deeper layers. In the upper layers barium and zinc are present in the form of oxide, and in the lower ones in the form of carbonate."

So, are barium and zinc more hazardous than lead? “Lead is one of the most toxic metals, but it is retained in the soil and this reduces its danger,” replies Carrero.

In the thesis, steps have also been taken to analyse the concentration of metals in roadside plants, a line that Carrero will be continuing to do research on from now onwards: “We are going to study the isotopic relationship of the lead in these plant families, because we have seen that it is possible to determine the origin of the lead (whether it is anthropogenic or natural)."